1. Field of the Invention
This invention relates to the sound reproduction and more particularly, to loudspeaker diaphragms of the type which are made of sintered ceramics.
2. Description of the Prior Art
As is well known in the art, dome-shaped loudspeakers usually comprise a diaphragm with an outer peripheral edge portion, a voice coil assembly adhered to the outer peripheral edge portion at an upper peripheral edge thereof, and an edge adhered also to the outer peripheral edge portion along the tip thereof, thereby permitting on-center mounting. This diaghragm system is set in a magnetic circuit made of a pole piece and a top plate.
With loudspeakers using a dome diaphragm of the justmentioned type, larger high resonance frequency, f.sub.H, which initially produces a peak on an output sound pressurefrequency characteristic, results in a more extended upper frequency limit, permitting a wider usable range of frequency. It is generally accepted that if a thickness of the dome and a ratio of weights of the dome and a voice coil assembly are constant, the following empirical formula is established with respect to the high resonance frequency, f.sub.H, ##EQU1## in which H is a height of the dome, D is a diameter of the dome, E is a Young's modulus of a dome material, .rho. is a density of the dome material, and .sqroot.E/.rho. is a sound velocity of the dome material. The above formula demonstrates that a higher sound velocity of the dome material results in a higher f.sub.H value, with better results.
Known dome-shaped diaphragms are usually made of light metals such as aluminium, titanium and the like, resinimpregnated woven fabrics, and plastics such as polypropylene, polycarbonate and the like. The Young's moduli and sound velocities of these materials are very low as particularly indicated in Table 1 appearing hereinafter. Accordingly, high resonance frequencies cannot be expected using these materials, with a narrow usable range of frequency.
In contrast, aluminium oxide, which is typical of ceramic materials, has, for example, a Young's modulus about 8 times larger and a sound velocity about two times larger than those of metallic aluminium. The resonance frequency can be made higher by about two times as will be seen from Table 1. In other words, where a loudspeaker is constituted of a ceramic such as aluminium oxide, the upper frequency limit can be much more extended than will be expected from metallic aluminium or plastic materials.
TABLE 1 __________________________________________________________________________ Young's Modulus Density Sound Velocity Rigidity .times. 10.sup.10 Index Index Index Index Material [Pa] to Al g/cm.sup.3 to Al Km/s to Al Eh.sup.3 to Al __________________________________________________________________________ Aluminium 7 1 2.7 1 5.1 1 3.6 1 Titanium 11 1.6 4.5 1.7 4.0 0.96 1.2 0.3 Poly- 0.05 0.9 0.3 0.75 0.15 0.7 0.2 propylene Paper 0.05-0.3 0.3-0.7 1-2.3 1.9-8.3 Single 52 7.9 3.95 1.5 11.4 2.2 8.4 2.3 crystals of alumina Poly- 38 5.3 3.9 1.4 9.9 1.9 6.4 1.8 crystals of alumina __________________________________________________________________________
Other factors which give great influence on acoustic characteristics of loudspeaker diaphragm include the weight of the diaphragm. The weight has a great relation to the efficiency of converting electrical signals into sound. Although diaphragms should be generally light in weight, a magnetic circuit of a specific type allows use of a diaphragm material which is about two times as heavy as aluminium with respect to density. However, higher densities bring about several disadvantages such as a lowering of sound pressure and a deterioration of frequency response of the diaphragm. Especially, in the case of super tweeter which is used in the highest frequency range, better acoustic characteristics are obtained when the sound velocity is higher, and the weights of the diaphragm and voice coil assembly are smaller.
Metallic aluminium dome-shaped diaphragms which are currently employed usually have a thickness of about 30 microns, whereas when aluminium oxide is used to make a dome-shaped diaphragm, its thickness inevitably exceeds about 100 microns and thus a diaphragm having a thickness of about 30 microns cannot be obtained. This leads to a weight of about 4 to 5 or more times greater than the weight of a metallic aluminium dome-shaped diaphragm, resulting in a lowering of sound pressure and a deterioration of frequency response of the diaphragm.
Moreover, fabrication of loudspeaker diaphragms using ceramic materials such as aluminium oxide essentially requires a firing process. This may cause deformation in the shape of the diaphragm during or after the firing which will not be experienced in the fabrication using metallic aluminium. Alternatively, the diaphragm may become irregular on the surfaces thereof after firing, resulting in breakage of the diaphragm at the time of assembling of a loudspeaker.